Technical overview

General premise

Telplin uses both the untyped and the typed syntax tree to construct a signature file.

The implementation file is parsed with Fantomas's own parser (Fantomas.FCS) and transformed to its Oak model. Telplin walks the Oak and constructs the signature counterpart of every node: a BindingNode becomes a ValNode, for example.
The signature tree is constructed as close as possible to the implementation tree, and you'd be surprised how much information can just be re-used going from one to the other.

When the Oak does not contain sufficient information (an inferred return type, say), a FSharpSymbol is queried from the typed tree to fill in the gap.
The finished signature Oak is printed by Fantomas, so the output is formatted source code from the start.

Solution structure

graph TD D[Telplin.Core] E[Telplin] F[Telplin.Lambda] G[Telplin.Core.Tests] D --> E D --> F D --> G H[Telplin.Deploy] I[OnlineTool]

Telplin.Core

Has a reference to Fantomas.Core and processes the untyped tree using the Oak model. Note that Fantomas does not use FSharp.Compiler.Service: it ships its own parser, Fantomas.FCS.

Also references the FSharp.Compiler.Service, which is used to type check the implementation files.
The untyped tree constructed by the FSharpChecker is very deliberately not used, due to its incompatibility with Fantomas.

Telplin

The command line tool. It resolves the input (a source file, a project, a folder or a response file), runs a design time build through MSBuild to obtain the compiler arguments, and orchestrates the run described on the start page: generate, verify, write, clean up the implementation files and list the signatures in the project file.

Telplin.Lambda

An AWS lambda that powers the back-end of the online tool.

Telplin.Core.Tests

An NUnit unit test project that verifies the core logic.

OnlineTool

A Fable application that powers the front-end of the online tool.

Telplin.Deploy

A Pulumi program that deploys the online tool's infrastructure to AWS.